In what follows, gas turbines are taken as reference thermal equipment and the following terminology is adopted:                the term “combustion fluid” refers to any liquid or gaseous fluid which is introduced into the combustion chamber of the thermal equipment, which category includes: (i) liquid or gaseous fuels and, by extension, fuels employed in particulate form, such as pulverized coal or sawdust, which may be injected under pressure into the combustion chambers and the physico-chemical characteristics of the combustion of which are similar to those of fluid fuels; (ii) the oxidizer (or combustion air) and (iii) certain auxiliary fluids such as: atomizing air, used to spray the liquid fuel into the combustion chamber; a fluid possibly used to introduce a combustion additive; water or steam possibly used to obtain particular effects, such as a reduction in NOx emissions (“deNOx steam or water”) or an increase in power of the turbine;        the term “firing temperature” of a gas turbine refers not to the temperature within the flames but that of the combustion gases at their entry into the expansion turbine. This is a particularly important design parameter of gas turbines as it determines the maximum efficiency thereof;        the “hot parts” of thermal equipment are the components placed in the hot gas stream and comprise, in particular: the walls of the combustion chamber and its components, the combustion gas ducts and, depending on the type of equipment, the expansion turbine, the cylinders and pistons, tubes and walls of heat exchangers, etc. For example, in third-generation gas turbines, the firing temperature of which may reach 1400° C., the hottest parts are the components of the combustion system and of the expansion turbine, and mainly the parts of the first stage: stationary blades (“inlet guide vanes”) and moving blades (“rotor blades”). Metallurgical constraints dictate that these components be internally cooled so that their skin temperature (i.e. the temperature of their walls) is below that of the combustion gases. In general, the temperatures to which the walls of the hot parts are exposed depend on many factors, among which are the type of equipment, its design (types of materials; presence or absence of internal cooling and surface heat shields) and its operating conditions. The temperature level may be considered to be between 800 and 950° C.;        “traces” denote residual concentrations of certain elements introduced into the combustion chamber;        “corrosive metals” comprise, in the present context, alkali metals (in particular Na and K) and lead, the ash of which causes, as primary effect, high-temperature corrosion of metal alloys; their ash and their deposits will also be termed “corrosive”;        “fouling metals” or “Mf” essentially comprise alkaline-earth metals (particularly: Ca and Mg) and certain transition metals (particularly Cu, Ni and Zn), the ash of which tends to be deposited on the hot parts and to cause, as primary effect, fouling phenomena, i.e. the appearance and progressive growth of layers of hard deposits adhering to the hot parts; their ash and their deposits will also be termed “fouling”; lead also belongs to the category of fouling metals; and        “deleterious metals” comprise corrosive metals and fouling metals; their ash and their deposits will also be termed “deleterious”.        
In thermal equipment, the combustion fluids often contain traces of certain deleterious metals which threaten, depending on the case, the metallurgical integrity of said equipment (in the case of corrosive metals), their energy performance (in the case of fouling metals) or both these aspects at the same time (in the presence of both types of metals). In particular, it should be stressed that the materials balance of deleterious metals must be considered within the combustion gases in which the hot parts are located or, which is equivalent, in the combustion chamber where the potentially contaminated combustion fluids are mixed together.
Among corrosive metals are alkali metals: traces of sodium and potassium are frequently conveyed by the combustion fluids, particularly by a liquid fuel or by “deNOx water”, the traces usually being in the form of chlorides. In the flames, these chlorides react with sulphur brought by the fuel or the ambient air, so as to form the corresponding alkali metal sulphates, namely Na2SO4 and K2SO4. In the molten state, alkali metal sulphates constitute electrolytic media that are highly aggressive with respect to metal alloys and are capable of corroding those portions of hot parts on which they are deposited and remain for a long enough time. The melting point (“Tm”) of pure Na2SO4 is for example 884° C., which temperature, as will be indicated below, may greatly decrease in the presence of other metal salts. In addition, it should be noted that liquid films of alkali metal sulphates are relatively fluid and therefore tend to flow along the walls under the effect of the gas streams that permanently sweep the hot parts. This reduces their chance of being lastingly attached to defined metal portions so that the cases of high-temperature corrosion are less frequent than if these films were to be stationarily attached to the same metal portions. Moreover, molten alkali metal sulphates have a non-negligible vapour pressure at high temperature, so that their deposition on the hottest parts (for example those of the first stage of an advanced-technology gas turbine) may be reduced by this vaporization phenomenon. However, in this case, sulphates are deposited on other parts, located further downstream, which are less hot.
The types of corrosion caused by alkali metal sulphates are usually called “type I” or “type II” high-temperature corrosion depending on their mechanism. In type I corrosion, the metal attack is uniform, with no pitting formation, whereas type II corrosion is characterized by the appearance of surface pitting and by the existence of an “induction period” before manifestation of the corrosion. This induction period corresponds to the pitting incubation time, which is shorter the more aggressive the medium and the lower the intrinsic resistance of the alloy, determined in particular by its chromium content.
Apart from the primary factors formed by the temperature and the alkali metal content in the combustion gases, other parameters are involved in type I or type II corrosion rates. Two factors are particularly important, as the Inventors have observed:                the SOx (i.e. SO2 and SO3) concentration in the combustion gases—increasing this concentration accelerates the corrosion and in particular shortens the induction period that precedes the appearance of pitting in type II corrosion; and        the type of alloy: in fact, two main types of superalloys are used in the construction of gas turbines, namely, on the one hand, alloys called “chromia-forming alloys” developed in the 1960s and 1970s and typically used in the form of polycrystalline alloys in first- and second-generation turbines, these chromia-forming alloys developing, on their surfaces, passivation layers rich in chromium oxide (Cr2O3) which protect, within certain limits, the substrate from attack by molten sulphates and, on the other hand, so-called “alumina-forming” alloys which were developed more recently and are being used increasingly often in the fowl of single-crystal alloys in third-generation gas turbines, which have very high firing temperatures, these alumina-forming alloys developing surface layers rich in alumina which give them, compared with chromia-forming alloys, enhanced hot refractory and mechanical properties, but they have a much lower resistance to high-temperature corrosion.        
With regard to fouling, several types of metals have effects to various degrees:                traces of calcium and magnesium generate calcium sulphate (anhydrite: CaSO4) ash and magnesium sulphate (MgSO4) ash, and also magnesium oxide (MgO), said traces generally coming from the fuel or from “deNOx water”—CaSO4 has a very pronounced fouling effect; and        traces of transition metals, which generate ash based on oxides and oxisulphates, may be provided by water contained in one of the combustion fluids. For example, the moisture contained in the combustion air or in the atomizing air could be in contact, upstream of the combustion chamber, with a galvanized or carbon steel structure, or else with a brass or cupro-nickel exchanger, and become laden with zinc, copper or nickel.        
Finally, lead occupies a particular position among the deleterious metals, it being possible for lead-containing ash to have both a fouling role and a corrosive role. Traces of lead that may be encountered, albeit quite rarely, in certain fuels derived from petroleum (naphthas, gasoils) generally derive from accidental contamination by lead-containing petrol. Depending on the conditions (sulphur content of the fuel, temperature, presence or absence of alkali metals, etc.), the lead-containing ash may be fusible (the most general case) or refractory:                pure lead sulphur (PbSO4) has a high melting point (Tm: 1075° C.) and is capable, when present in a high enough concentration, of generating fouling deposits;        nevertheless, depending on the temperature and the sulphur content of the fuel, PbSO4 usually undergoes a desulphurization process, to form a series of oxisulphates having decreasing melting points: (i) PbO—PbSO4 (Tm: 965° C.), (ii) 2PbO—PbSO4 (Tm: 932° C.), (iii) 4PbO—PbSO4 (Tm: 800° C.) and, finally, (iv) PbO (Tm: 885° C.). These phases are known for their very great corrosivity at high temperature.        
Lead is therefore a metal which is both very corrosive and potentially fouling.
The fraction of ash resulting from the fouling metals, and deposited on the hot parts, undergoes a sintering phenomenon by prolonged exposure to high temperature: the deposits thus tend to become both hard and adherent, which, combined with their insolubility in water (or even their possible tendency to agglomerate and harden upon contact with water) makes them very difficult to remove and justifies their being called fouling deposits. Their main effect is the degradation in the performance of the thermal equipment, which degradation progressively increases as they accumulate, thereby resulting in: (i) a reduction in heat exchange coefficients; (ii) a deterioration in the aerodynamics of the gas flows; and (iii) in extreme cases, a reduction in hot-gas flow passage areas. It may then prove necessary for the thermal equipment to be partially dismantled, so as to manually restore its state of cleanliness and its performance.
In reality, traces of corrosive metals and fouling metals are usually present simultaneously, and secondary deleterious effects may result from a combination thereof. Firstly, attachment of anhydrite CaSO4 particles on the walls is facilitated by traces, even minute traces, of molten alkali metal sulphates. Secondly, the sulphates resulting from fouling metals (Mf) often combine with those of alkali metals (Ma) to give double sulphates of the MfSO4-Ma2SO4 type or even triple sulphates of the MfSO4-Ma2SO4-Ma′2SO4 type, which hereafter will be called “complex salts”. These complex salts often have initial melting temperatures (or solidus temperatures) below the wall temperature of the hot parts, which enables them to adhere thereto.
For example:                CaSO4 (Tm˜1450° C.) forms, with Na2SO4 and K2SO4, complex salts of the (Na,K,Ca)(SO4) type, the three eutectics, two binary and one ternary, of which have the following respective compositions:        
50% Na2SO4-50% CaSO4 (Tm: 916° C.);
62% K2SO4-38% CaSO4 (Tm: 867° C.) and
31% Na2SO4-29% K2SO4-40% CaSO4 (Tm˜800° C.);                MgSO4 likewise forms three eutectics:        
56% Na2SO4-44% MgSO4 (Tm: 662° C.);
72.5% Na2SO4-27.5% MgSO4 (Tm: 750° C.),
13% Na2SO4-55% K2SO4-32% MgSO4 (Tm: 670° C.);                PbSO4 forms, with Na2SO4, the eutectic:        
53% Na2SO4-47% PbSO4 (Tm: 730° C.);                ZnSO4 forms the eutectic:        
45% Na2SO4-55% ZnSO4 (Tm: 472° C.);                NiSO4 forms the eutectic:        
62% Na2SO4-38% NiSO4 (Tm: 665° C.) and                CuSO4 forms the eutectics:        
53% Na2SO4-47% CuSO4 (Tm: 537° C.) and
55% K2SO4-45% CuSO4 (Tm: 460° C.).
It should be noted that, although magnesium, lead and transition metal (Cu, Zn, Ni) sulphates are of limited thermal stability and decompose into oxisulphates or oxides at high temperature, they are stabilized when they are combined with alkali metal sulphates in the above complex salts. Moreover, these combinations reduce the vapour pressure and consequently the vaporization of alkali metal sulphates and therefore tend to accentuate their corrosive effect. Finally, whereas molten alkali metal sulphates are very fluid in the pure state and tend to flow along the hot walls, these complex salts are in general viscous or even have a “pasty” consistency. Consequently, not only do they become attached more easily to the metal walls and participate in the fouling and corrosion processes, but they also play in turn the role of “adhesive” with respect to various liquid or solid particles that may strike said walls. This leads to local build-up of scoria and to aggravated fouling and/or corrosion effects. To summarize, there are secondary deleterious effects which may be termed “synergistic” and which result from physico-chemical interactions between the ash of the various deleterious metals. While alkali metal sulphates play the role of fluxing agents with respect to particles containing fouling metals, the sulphates of fouling metals play the reverse role of “thickeners” with respect to the first sulphates. Lead may itself play both these roles or act in combination with the other deleterious metals.
In what follows, “oxisulphate-containing ash” will be used, for the sake of simplification, to denote all of the following compounds: sulphates, oxisulphates, complex salts and oxides of deleterious metals present at the combustion chamber exit.
Among the drawbacks that have been described above, high-temperature corrosion has the most dangerous effects as it may lead to hot parts fracturing, which occurs not only unexpectedly but may also cause, in the case of rotating machines, a cascade of destructions resulting in very expensive repairs and lengthy down-times of the equipment.
Lead contamination is rare but may be substantial, and at the present time there exists no method of inhibiting lead-induced high-temperature corrosion.
The other deleterious metals are practically always present in the trace state and their strict absence is very difficult to ensure under industrial exploitation conditions of thermal equipment. For example, even if the operator could provide, for a certain extra-cost, a gasoil free of metal contamination, especially free of sodium, this level of purity cannot be guaranteed during the subsequent transportation phases (the problem of keeping pipelines, wagons or tankers clean) and may also become degraded in the storage tanks of the power station through the tank “breathing” effect (ingress of air as a result of variations in level and in day/night temperature cycles).
From another standpoint, biofuels consisting of either a primary biomass (wood, straw, agricultural production residues, vegetable oils obtained by trituration of oleaginous grains, etc.) or a secondary biomass (ethanol, vegetable oils methyl esters (VOME), etc.), which biomasses are having to be developed as substitutes, or as complementary fuels, for fossil fuels in various types of thermal equipment, are very often contaminated with metal. For example, in the case of a primary biomass and VOME biofuels, calcium is mainly found as fouling metal and potassium as corrosive metal. Even if the biofuels have a very low sulphur content owing to their plant origin, their sulphur contents, of the order of a few mg/kg, which are added to the residual sulphur contents provided by the ambient air, are in general sufficient to produce oxisulphate-containing ash in the flames. As a consequence, biomass combustion has potentially deleterious effects on thermal equipment, which are similar in their nature to those of fossil fuels, but with potentially more intense effects because of the higher original metal contamination levels.
There is therefore an increasing conflict between, on the one hand, the requirement—dictated by the design of thermal equipment with the highest performance, including modern gas turbines—to supply very pure and therefore expensive fuel and, on the other hand, the need to maintain operating costs at a sustainable level.
Moreover, because of the ever lower levels of metal contamination required by gas turbines of the latest generation because of their increasing firing temperatures, the problems described above may affect not only units burning liquid fuels but also those which burn very pure commercial gases and that are installed in marine or industrial atmospheres, i.e. in salt-contaminated environments, knowing that the techniques currently available for removing these salts from the air are very expensive and their efficiency is tricky to control.
With regard to corrosion by alkali metals and more particularly by sodium, the patents FR 1 207 857 from Continental Oil (1959) and U.S. Pat. No. 3,018,172 from Tillman (1962) on the one hand teach the beneficial effect provided by aluminum compounds—aluminum being termed in what follows an “active” metal—over trial times of 50 hours and, on the other hand, disclose Al/Na doses of between 2 and 6 by weight. However, the Inventors have observed that such a protection method has shortcomings in a great number of situations:
(a)—either in the case of durations greatly longer than 50 hours, corresponding in fact to industrial operating conditions of the equipment. For example, in one of the trials carried out by the Inventors, test specimens made of a nickel-based superalloy GTD111 were kept in prolonged contact at 900° C. with a sodium sulphate bath to which alumina had been added in an Al/Na mass ratio of 2.5, which specimens, after 200 hours, suffered intense pitting corrosion. This aspect could not have been observed by Tillman, the test times for which did not exceed 50 hours, which is shorter than the pitting induction period prevailing under these conditions;
(b)—or in the presence of fouling metals tending to form “complex sulphates” with alkali metal sulphates according to the mechanisms described above. For example, in another trial carried out by the Inventors, the 50% Na2SO4/50% CaSO4 eutectic mixture (Tm: 916° C.) resulted, at a temperature of 950° C., in pitting of the GTD111 test specimens more rapidly than in the absence of calcium, for the same Al/Na dosage; moreover, a highly viscous layer of the eutectic was observed to form at high temperature around the part, which, after cooling, left a highly adherent coating;
(c)—or in the presence of high SOx contents in the combustion gases (due to the high sulphur contents of the fuel). In another series of trials carried out by the Inventors, an Al/Na mass dosage of 3, which had provided effective protection of the GTD111 superalloy at 900° C. over a duration of 400 hours, for a 50 ppm SOx content, allowed pitting to develop after about 300 hours when this content was adjusted to 200 ppm; and                (d)—or, finally, in the case of alumina-forming superalloys. For example, the Inventors have observed that an Al/Na dosage of 3, which correctly protected the GTD111 alloy (containing 14% Cr and 3% Al), at 900° C., over a period of 400 hours and in the presence of 50 ppm of sulphur, gave poor results with the alumina-forming alloy N5 (containing 7.6% Cr and 6% Al) under the same conditions.        
The Inventors have also observed that the sodium sulphate used during the tests is unaltered at the end of the trials, which indicates the absence of a reaction between the sulphate and the alumina used or the aluminum salt used (which releases alumina when hot).
To summarize, the Inventors' analysis of the close links between the fouling and corrosion processes at high temperature shows that it is not a question of seeking to remedy the two effects independently, but rather, on the contrary, of having to address the causes of both problems simultaneously. This simultaneous approach would be a novel approach for maintaining integrity and performance in thermal equipment.
Moreover, the work carried out by the Inventors clearly confirms the beneficial effect of aluminum in the high-temperature corrosion by alkali metal sulphates. However, it also shows that long-term protection is very difficult to achieve. In particular when the corrosive conditions of the trial are exacerbated (using either a higher SOx content or a corrosive medium containing a fouling metal) or else when a less-resistant superalloy is used, the aluminum dosage must be increased and it is difficult to define a dosage which provides protection over a long period, knowing that the corrosion trials to be carried out then become very long (and expensive) owing to an increase in the pitting induction period. The results of these studies also suggest that the permanence of the risk of pitting corrosion may be linked to the absence of a chemical reaction between the aluminum and the alkali metal sulphate, which therefore retains its corrosive properties.
Finally, no method exists at the present time for modifying the ash that enables the fouling effects of alkali-earth and transition metals to be eliminated.